Method of gluing a plurality of wooden elements comprising curing a urea formaldehyde adhesive by means of high-frequency heating



,part of Serial No. 100,765, filed June Patented Apr. 8, 1952 METHOD OFGLUING A PLURALITY. OF

WOODEN ELEMENT S COMPRISING CURING. A UREA FORMALDEHYDE AD- HESIVE BYQUENCY HEATING Elmer E. Bergey,

MEANS OF HIGH-FRE- Lansdale, Pa., assignor to National Casein Company,Chicago, 111., a corporation of Illinois No Drawing. Application June 9,1950, Serial No. 167,250

3 Claims.

The object of the invention is to provide an improved hardener for usewith urea-formaldehyde resins in the edge gluing of wood in particular,and other materials to a substantial degree, by means of high frequencyelectric currents,

now abandoned.

In recent years many new and remarkably improved gluing techniques havebeen developed especially for use in the woodworking industry. and ofthese probably the most impressively successful has been that whichinvolves the use of high frequency energy in curing heat-sensitiveglues, as ameans of both enhancing the quality of laminated woodproducts and in effecting substantial operating economies.

The principal methods by which high frequency energy is applied to theunification or gluing of laminated wooden bodies are known asperpendicular heating and parallel heating, and ineach case it is of theutmost. importance that proper adhesives be used, and that these shallharden or set as quickly as possible. In perpendicular heating the highfrequency field is perpendicular to theglue laminations or veneers andthe intervening glue joints are heated simultaneously. This method isused in the production of both flat and curved plywood, laminated wood,furniture panels and the like. is applied to the union of multiplelaminations and intervening glue layerswherein the high frequency fieldis parallel with the glue. lines, which latter are. virtually the onlypart of the load that is heated even momentarily.

While it is true that both the wood and the intervening glue jointscompete as conductors of the high frequency energy, the glue iscomparatively much more conductive thanthe wood, and consequentlyconducts much more of the electric energy and is correspondingly heatedto a much greater degree. Parallel heating is the more efficient method,due to the fact that the mass of the glue heated is relatively verysmall when compared with the bulk of. the wood being bonded, However,the glue requirements. for parallel heating are far more exacting thanfor perpendicular heating, as the glue must be of a heat-sensitive type,and must be just sufiithis application being a continuation-im;

lines, so that both the wood The term parallel heating. by contrast.

ciently conductive to pass the high frequency current, while notsufficiently conductive to cause an arc to be produced.

As the wet relatively highly conductive glue conducts or passes theelectric current, it is caused to boil very quickly, usually within oneor two seconds, and this condition continues to a diminishing degree aslong as the moisture of the glue is decreasing, and in fact until itdries out completely and its conductivity becomes practically nil, or atbest but slightly more conductive than the wood, which in the meantimehas been but very slightly heated. The so-called curing or setting of'the glue isa combination of drying and polymerization of the molecules,depending largely upon the type of glue that is used.

Of the adhesives developed and used thus far in parallel heating,urea-formaldehyde resins have produced the best results. However,numerous problems are encountered with any glue, some being totallyunadapted for use in parallel heating, as for example inherently lowconductivity glues. It must therefore be a thermosetting glue of justthe proper degree of conductivity to pass the electric current, and toset during the brief period during which it is subjected to the electricenergy ofhigh frequency. If it is too highly conductive, arcing takesvplace with resultant burning of the wood, whereas if it is too low inconductivity it will. not conduct the current sufficiently to producethe desired set, and at the same time the wood may be. heated to anundesirable degree. Likewise, the frequency of the high tension currentmust be. considered, for lower frequencies tend to heat the glue linemore than the higher frequencies when the same power goes into the load.

The primary object, therefore, is to provide a hardener for glues oradhesives of the ureaformaldehyde type that will aid the setting of theglue during the brief period of high frequency application, and whichwill insure the proper degree of conductivity in the glue line.

For this purpose it has been found that a combination of an ammoniumsalt of a strong acid, such as ammonium chloride, ammonium sulphate,ammonium nitrate, ammonium bromate, ammonium fluoride, etc., incombination with an alkaline earth phosphate, such as tricalciumphosphate, tribarium phosphate, tristrontium phosphate, trimagnesiumphosphate, trizinc phosphate, etc., or one of the said alkaline earthsalts of phosphoric acid, produces the desired degree of conductivityand speed of set required together with all of the other chemical andphysical characteristics desirable in a liquid ureaformaldehyde resinadhesive. It should also be understood that for practical purposes it ispossible to use any one of the said groups of ammonium salts incombination with any one of the said alkaline earth phosphates, or thereaction products of said alkaline earth salts with phosphoric acid.

The resin used is a water-soluble condensation product of urea andformaldehyde, that it is essential that it shall not have beenpreviously dried and remixed with water. Any resinforming urea may beused, c. g., urea or thiourea. Any aldehyde may be used, e. g.,formaldehyde, acetaldehyde or furfural. The resin most widely used bythe industry is a condensation product of urea (carbamide) andformaldehyde.

The urea (1 mol) is reacted under proper conditions with between 1.7 and2.5 mols of 37% formaldehyde, and the resin concentrated by vacuumdistillation to about 60 to 70% solids. The remainder is water and somefree formaldehyde. The ureaaldehyde resin may be modified with otheraldehyde reactive substances to form resin, e. g., resorcinol, aniline,xylenol, and the like.

Any commercial high frequency generator may be employed, though theyvary quite widely in electrode voltages and frequency, the latterranging between one and fifty megacycles, wherefore the proportion ofthe hardener to the quantity of glue used depends largely upon theparticular generator used. Some generators that employ higher voltagesacross the load tend to are with high proportions of the compositehardener, while with the characteristically lower voltages of othergenerators higher proportions of the improved hardener produce adefinite advantage by concentrating more power in the glue lines.

Formula I .An example of a glue that is best adapted for use withgenerators of the lowest has given best results with most generators ofintermediate power (i. e., in the neighborhood of 6 to 15 kw.)

100 parts liquid water-soluble urea-formaldehyde condensation product.

2 parts of an ammonium salt selected from the group herein beforelisted.

4 parts of an alkaline earth phosphate selected from the grouphereinbefore listed, or a reaction product of one or the same withphosphoric acid.

10 parts filler (omitted if desired).

Formula IV.An example of a glue that is best adapted for use withgenerators of somewhat higher than average power:

100 parts liquid water-soluble urea-formaldehyde condensation product.

1 part of an ammonium salt selected from the group hereinbefore listed.

1 part of an alkaline earth phosphate selected from the grouphereinbefore listed, or a reaction product of one of the same withphosphoric acid.

' 10 parts filler (omitted if desired).

Formula V.An example of a glue that is best practical power (i. e., inthe neighborhood of c Formula II .An example of a glue that is bestadapted for use with generators of somewhat lower than average power:

urea-formalde- 100 parts liquid water-soluble urea-formaldehydecondensation product.

5 parts of an ammonium salt selected from the group herein beforelisted.

3 parts of an alkaline earth phosphate selected from the grouphereinbefore listed, or a reaction product of one of the same withphosphoric acid. 10 parts of filler (omitted if desired).

Formula III.An example representing approximately proportions of a gluethat thus far practical power (i. e., 25 to 40 kw.)

parts liquid water-soluble urea-formaldehyde condensation product.

part of an ammonium salt selected from the group hereinbefore listed.

5 part of an alkaline earth phosphate selected from the grouphereinbefore listed, or a reaction product of one of the same withphosphoric acid.

10 parts filler (omitted if desired).

The curing properties of a particular glue formula of this nature willbe found to vary with the power of the generator, roughly between kw.and 40 kw., above which latter figure it is not practical to go, whilethe average usually vary between 6 and 15 kw. However, the curingdepends upon a number of variables such as frequency and voltage as wellas the capacity of the generator, while the power required is obviouslyincreased by an increase in the number, thickness, longitudinalmeasurement between electrodes and the area of the glue lines.Commercial generators in use today employ from 1200 to 4000 volts at theelectrodes, while the frequencies most often employed vary from 5 to 7megacycles, although as high as 25 megacycles have been used.

In considering the foregoing, it is to be understood that the liquidurea-formaldehyde condensation product resin should be in its initiallyprepared state of physical condition, and not previously dried andsubsequently mixed with water, since a resin of this nature has beendefinitely found to give markedly better results than when the resin hasbeen dried, or permitted to dry, and then at some future time mixed withwater.

While the chemistry of the reaction that takes place in the combinedurea-formaldehyde resin and hardener may not be fully understood at thistime, it is believed that there occurs a double decomposition type ofreaction, probably beginning with the addition of the hardener to theliquid resin, followed by further reaction upon their beingsimultaneously subjected to the effects of the high frequency current.In any case, a chemical reaction definitely takes place and is completedupon the setting of the glue as hereinbefore described.

An additional. advantage of the improved hardener described hereinresides in the fact, that in combination with urea-formaldehyde resinsit prevents the formation or development of. too greata degree ofacidity in the glue, orin other words it functions as an aciditycontrol, since too high a degree of acidity is definitely known to bedetrimental to the durability of the bond. The alkaline earth phosphatesassist by absorbing the excess acid.

Practically innumerable tests and experiments have been carried out inan effort to discover or develop the most efficient proportions of bothhardener per se, and of the combined liquid adhesive and dry hardener,and it has been found that the above formulas may be variedconsiderably, depending largely upon the electrical characteristics ofthe high frequency current and the makeup or composition of the baseadhesive, but that under the conditions set forth the specifiedproportions seem to produce the best results. Obviously manymodifications and variations may be made without departing from thespirit and scope of the invention.

During the research work that preceded the perfection of thehereinbefore described formulas, and the determination of theapproximate range of alternatives among the many available ammoniumsalts of strong acids .and alkaline earth phosphates, a great manyexperiments were made and tests run, of which the following areillustrative. Experiment I shows that certain ammonium salts used ascatalysts in urea formaldehyde resin adhesives do not act or functionindependently, but depend for their activity upon the presence of theformaldehyde in the resin, and also that the action with respect toammonium chloride differs from the action with ammonium silicofiuoride.

EXPERIMENT I [No formalin (HCHO) in Nos. 1 and 2.]

'6. EXPERIMENT II Water Parts 0.65 Powdered urea resin g 100'. Ammoniumsilicofluoride heated to 73 F g 2.

Gelling at 18 minutes.

Firm gel at 20 minutes.

Water parts 0.65 Powdered urea resin g 100. Ammonium chloride heated to73 F g 2.

Good flow at 32 minutes (pH 4.80). Usable flow at 128 minutes (pH 4.20).Very heavy at 155 minutes.

Beyond use at 178,minutes.

Working life, 2% to 3 hours.

Resin used was powdered urea-formaldehyde resin without catalyst.

Having thus described my invention, what I claim and desire to protectby Letters Patent of the United States is:

1. The method of edge-gluing a plurality of wooden elements whichconsists in providing a pair of elements with an intervening layer ofadhesive consisting of an aqueous urea-formaldehyde resin in itsoriginal liquid state and containing about 60 to 70% urea resin solids,and a hardener composed of an ammonium salt of a strong acid selectedfrom the group which consists of ammonium bromide, ammonium chloride,ammonium fluoride, ammonium nitrate and ammonium sulphate, and analkaline earth phosphate, selected from the group which consists oftribarium phosphate, tricalcium phosphate, trimagnesium phosphate,tristrontium phosphate and trizinc phosphate, clamping said elementstogether tightly, and then passing a high frequency electric currentthrough and in the plane of said adhesive to cure the same.

2. The method of edge-gluing a plurality of wooden elements whichconsists in providing a pair of elements with an intervening layer ofadhesive consisting of approximately 100 parts of an aqueousurea-formaldehyde resin in its original liquid state and containingabout to 70% urea resin solids, and a hardener composed of betweenone-half and eight parts of an ammonium salt of a strong acid selectedfrom the group which consists of ammonium bromide, ammonium chloride,ammonium fluoride, ammonium nitrate and ammonium sulphate, and betweenone-half and Water cc 30 Ammonium silicofluoride g 2 pH at 5 minutes3.72 pH at 70 minutes 3.72 pH at 135 minutes 3.72 No. 2

Water cc 30 Ammonium chloride g 2 pH at 5 minutes 3.50 pH at '70 minutes3.50 pH at 135 minutes 3.50 No. 3

Water cc 30 Ammonium silicofluoride g 2 Formaldehyde (37%) cc 2 pH at 5minutes 3.68 pH at 70 minutes (added additional 5 cc.

formaldehyde, 37%) 2.70 pH at 60 minutes 1185 No. 4

Water cc 30 Ammonium chloride g 2 Formaldehyde (37%) cc 2 pH at 5minutes 3.25 pH at 70 minutes (added additional 5 cc.

formaldehyde, 37 2.00 pH at 60 minutes -1 1.30

eight parts of an alkaline earth phosphate selected from the group whichconsists of tribarium phosphate, tricalcium phosphate, trimagnesi-umphosphate, tristrontium phosphate and trizinc phosphate, clamping saidelements together tightly, and then passing a high frequency electriccurrent of between /2 kw. and 40 kw. at a frequency of between 5 and 25megacycles through and in the plane of said adhesive to cure the same.

3. The method of edge-gluing a plurality of wooden elements whichconsists in providing a pair of element with an intervening layer ofadhesive consisting of approximately parts of an aqueousurea-formaldehyde resin in its original liquid state and containingabout 60 to 70% urea resin solids and a hardener composed of 7 from /2to eight parts ammonium chloride and UNITED STATES PATENTS from to eightparts of tricalcium phosphate Number Name Date clamping said elementstogether tightly, and 2019 834 viernng N0 5 1935 then passing a. highfrequency electric current 2236184 Menger 1941 through and in the planeof said adhesive to cure 5 2312210 Bearing 1943 the same 2,372,929Blessing Apr. 3, 1945 ELMER BERGEY- 2,399,980 Bradshaw May 7, 19462,413,624 Harris Dec. 31, 1946 REFERENCES CITED 2,453,135 Bilhuber Nov.9, 1948- The following references are of record in the 10 2,513,338simons 8 1950 file of this patent:

1. THE METHOD OF EDGE-GLUTING A PLURALITY OF WOODEN ELEMENTS WHICHCONSISTS IN PROVIDING A PAIR OF ELEMENTS WITH AN INTERVENING LAYER OFADHESIVE CONSISTING OF AN AQUEOUS UREA-FORMALDEHYDE RESIN IN ITSORIGINAL LIQUID STATE AND CONTAINING ABOUT 60 TO 70% UREA RESIN SOLIDS,AND A HARDENER COMPOSED OF AN AMMONIUM SALT OF A STRONG ACID SELECTEDFROM THE GROUP WHICH CONSISTS OF AMMONIUM BROMIDE, AMMONIUM CHLORIDE,AMMONIUM FLUORIDEM AMMONIUM NITRATE AND AMMONIUM SULPHATE, AND ANALKALINE EARTH PHOSPHATE, SELECTED FROM THE GROUP WHICH CONSISTS OFTRIBARIUM PHOSPHATE, TRICALCIUM PHOSPHATE, TRIMAGNESIUM PHOSPHATE,TRISTRONTIUM PHOSPHATE AND TRIZINC PHOSPHATE, CLAMPING SAID ELEMENTSTOGETHER TIGHTLY, AND THEN PASSING A HIGH FREQUENCY ELECTRIC CURRENTTHROUGH AND IN THE PLANE OF SAID ADHESIVE TO CURE THE SAME.